Field of the Invention
The present invention relates to a toner for developing an electrostatic image to be used in image forming methods such as electrophotography and electrostatic printing.
Description of the Related Art
As an electrophotographic apparatus using a toner, there are given a laser printer and a copying machine. In recent years, colorization has advanced rapidly, and hence there is a demand for further increase in image quality.
As one of the problems of the electrophotographic apparatus using a toner, first, there is given fogging. In a development process, a toner is developed also in a non-image portion, and a portion in which an image is not intended to be formed is colored. Such an image defect is called fogging.
It is considered to be very difficult to completely eliminate the generation of fogging, that is, to reduce the amount of a toner that is developed in the non-image portion to zero. Meanwhile, it is possible to reduce fogging to an invisible degree. Therefore, hitherto, there have been various proposals regarding means for suppressing fogging. Those technologies basically involve reducing fogging to an invisible degree. In particular, there is given a procedure involving controlling the charge quantity of a toner.
The main cause for the development of a toner in the non-image portion is that particles of the toner contain a particle having an insufficient charge quantity and a particle charged to opposite polarity. The toner having an insufficient charge quantity is slow to react to a back contrast and is transferred to the non-image portion stochastically or due to the action of adhesive force other than electrostatic force. The back contrast refers to a potential difference that is formed between the potential of a toner bearing member and the potential of an electrostatic latent image-bearing member (photosensitive member) in the non-image portion so as to prevent a toner from being developed in the non-image portion to the extent possible. Further, the toner charged to opposite polarity is actively developed in the non-image portion. In order to achieve a toner having those inconvenient particles suppressed to the extent possible, various technologies regarding a toner have been proposed.
As a method of controlling the charge quantity of a toner, there is given a method involving causing an external additive, such as silica fine particles, to adhere to the surface of a toner particle to ensure flowability, thereby uniformizing charging. However, in the case where an image is printed on a large number of sheets, the external additive is embedded or detached, and hence the method still remains susceptible to improvement in terms of fogging. As an improving method therefor, a method has been considered, which involves uniformly covering the surface of a toner particle with a silicon compound.
In Japanese Patent Application Laid-Open No. H03-089361, as the method involving covering the surface of a toner particle with a silicon compound, there is a disclosure of a method of producing a polymerized toner involving adding a silane coupling agent to a reaction system.
Further, in Japanese Patent Application Laid-Open No. H09-179341, there is a disclosure of a polymerized toner having on the surface thereof a coating film of a reaction product of a radical reactive organosilane compound.
Further, as another problem of the electrophotographic apparatus using a toner, there is given improvement of transferability. When a toner image formed on a photosensitive member is transferred onto a transfer material by a transfer unit, there is a case where a transfer residual toner remains on the photosensitive member. In this case, it is necessary to clean the photosensitive member by a cleaning device to recover the transfer residual toner into a waste toner container. However, due to the presence of the cleaning device and the waste toner container, the apparatus is increased in size, which becomes an obstacle for downsizing the apparatus. Further, in a cleaner-less system, it is also necessary to satisfy both a sufficient cleaning property and sufficient transferability for a long period of time, and hence it is considered necessary to remarkably highly control the surface shape of the toner particle.
Further, when a toner is transferred from the photosensitive member onto the transfer material, the amount of a toner that remains on the photosensitive member without being transferred onto the transfer member, that is, the transfer residual toner changes depending on the transfer current. In general, there is an optimum range of the transfer current in which the amount of the transfer residual toner becomes minimum. In the case where the transfer current is lower than the optimum current range, a transfer electric field is small relative to attraction force between the toner and the photosensitive member, and hence the toner does not move to increase the amount of the transfer residual toner.
Meanwhile, in the case where the transfer current is larger than the optimum current range, discharge occurs in a toner layer to rather decrease the transfer electric field, and hence the transfer residual toner is increased. Thus, it is desired that the transfer current be set to the lowest within the optimum current range.
However, the optimum current range changes also depending on the charge quantity of a toner. In particular, in the case where printing is not performed for a long period of time in a high-humidity environment, a reduction in charge quantity and a change in attraction force between the toner and the photosensitive member are liable to occur, and hence the optimum range of the transfer current is liable to change. In order to address this change, there is a method involving determining a transfer current by an environment detection device, such as a temperature and humidity sensor. However, there is a concern that various control devices may be complicated and increased in size. Therefore, there is a demand for a toner having satisfactory transferability within a wide transfer current range without a change in charge quantity even under high temperature and high humidity.
In view of the foregoing, in Japanese Patent Application Laid-Open No. 2002-108001, as a procedure for enhancing transfer efficiency, there is a disclosure of a toner having added thereto a spherical external additive having a large particle diameter.
Further, in Japanese Patent Application Laid-Open No. 2004-085850, there is a disclosure of a toner in which an irregularity period of the surface of a toner obtained by externally adding silica particles to toner particles pulverized by a jet mill is measured by a scanning probe microscope (SPM), and a large irregularity period and a small irregularity period are controlled. There is also a disclosure that, with the foregoing, the flowability of the toner is improved, and a uniform toner brush can be realized, to thereby obtain high image quality excellent in dot reproducibility.